The name Éire is the dative form in modern Irish Gaelic of the name for the goddess Eriu, a mythical figure who aided the Celts conquer Ireland as described in the Book of Invasions.

Since 1949, the term Republic of Ireland has generally been used in preference to Éire, to clarify that the country rather than the whole island is under discussion. It is sometimes felt that use of "Éire" is associated with a condescending attitude to Ireland in some right wing elements of the British media. Technically, however, as the Republic of Ireland Act enacted in 1948 makes clear, the 'Republic of Ireland' is actually a description not a name, even if generally used as such.

De Valera did have a history of making statements on constitutional matters that were legally questionable. His belief that the Governor-General's post had been abolished by a constitutional amendment in December 1936 was privately rejected by his own Attorney-General, James Geoghegan, Secretary to the Executive Council (ie, the state's main civil servant and his own closest advisor), Maurice Moynihan, the Parliamentary Draftsman's Office (which drafted legislation) and other leading legal figures in the government. To sort out what was privately seen as a legal mess, de Valera had had to introduce a second enactment, the Executive Power (Consequential Provisions) Act, 1937, which he retrospectively backdated to the original date of the supposed abolition in December 1936. In 1947, de Valera's new Attorney-General, future President of Ireland Cearbhall O'Dalaigh, began drafting a bill to grant to the President the powers in international affairs possessed by the King. Part of the debate in government revolved around whether a republic should be declared in the bill. The very existence of the debate is evidence that de Valera's latest attorney-general and part of his cabinet, maybe even de Valera himself, did not agree with De Valera's statement in 1945 that Éire was already a republic. In the end the draft bill was never submitted to the Oireachtas for approval. Whether that is because it was simply abandoned or because de Valera planned to introduce it after the 1948 general election (which he unexpectedly lost) is unclear.

A bill to finally and unambiguously declare a republic was introduced in 1948 by the new taoiseach, John A. Costello of the Fine Gael party. What caused the bill to be introduced remains a mystery. Costello made the announcement that the bill was to be introduced in Ottawa, during an official visit to Canada. It had been suggested that it was a spur of the moment reaction to offence caused by the Governor-General of Canada, who was of Northern Irish descent and who allegedly placed symbols of Northern Ireland, notably a replica of the famous Roaring Meg cannon used in the Siege of Derry, in front of an affronted Costello at a state dinner. What is certain is that the prior arrangement whereby toasts to the King (symbolising Canada) and the President (representing Ireland) were to be proposed, was broken. Only a toast to the King was proposed, to the fury of the Irish delegation. Shortly afterwards Costello announced the plan to declare the republic.

According to all the ministers in Costello's cabinet but one, however the decision to declare a republic had already been made prior to Costello's Canadian visit. Costello's revelation of the decision was because the Sunday Independent (an Irish newspaper) had discovered the fact and was about to 'break' the story as an exclusive. However one minister, the controversial Noel Browne gave a different account in his autobiography, Against the Tide. He claimed Costello's announcement was done in a fit of anger of his treatment by the Governor-General and that when he returned, Costello, at an assembly of ministers in his home, offered to resign because of his manufacture of a major government policy initiative on the spot in Canada. However, according to Browne, all the ministers agreed not to accept the minister and to manufacture the story of a prior cabinet decision.

The evidence of what really happened remains ambiguous. There is no record of a prior decision to declare a republic before Costello's Canadian trip, among cabinet papers for 1948, which supports Browne's claim. However, in what is generally regarded as one of its most ill-judged decisions, the Costello government refused to allow the Secretary to the Government, Maurice Moynihan, to attend cabinet meetings and take minutes, because they believed he had been close to their enemy, Eamon de Valera (as de Valera had been in office continually for sixteen years and directly preceded them, and as Moynihan had been the state's chief civil servant for much of that time, it was hardly surprising that he would have been close to de Valera. No evidence suggests however that he was pro-de Valera and anti-them, and they reversed their decision when they returned to government in 1954.) As a result the minutes were kept by a Parliamentary Secretary (junior minister), future taoiseach Liam Cosgrave. As someone who had never kept minutes before, it is understandable that Cosgrave's minutes at least early on in the government proved less than a thorough record of government decisions. So whether the issue was never raised, was raised but no decision taken, a decision taken informally or formally cannot be unambiguously clarified on the basis of the less than adequate minutes of cabinet meetings in 1948.

In addition, Browne's own book, published in the 1980s, is littered with major factual inaccuracies and thus is seen as equally unreliable. The last two surviving ministers of that cabinet in the 1980s, former Minister for External Affairs Sean MacBride and Browne, publicly and trenchantly disagreed as to the events that led to the declaration of the republic. What is certain is that one man's account is wrong. But it has proved impossible to ascertain for certain which one's.1

The Republic of Ireland Act was enacted in Oireachtas Éireann with all parties voting for it. De Valera did suggest that it would have been better to reserve the declaration of the republic until Irish unity had been achieved, a comment hard to reconcile with his 1945 claim that Éire was already a republic. Speaking in Seanad Éireann Costello told senators that as a matter of law, the King was indeed 'King of Ireland' and Irish head of state and the President of Ireland was in effect no more than first citizen and a local notable, until the new law came into force.

On 1 April1949 the Republic of Ireland Act, 1948 came into force. Ireland ceased to have a king. The President of Ireland was upgraded to a full head of state. While the constitutional name of the state, Éire was not changed, the descriptive name given to Éire in the new Act, The Republic of Ireland, became the effective name of the twenty-six county state. All previous ambiguities over name, title, head of state and the positions of the King of Ireland and the President of Ireland were resolved. The Westminster Parliament passed its own Ireland Act 1949 acknowledging the changes, preserving certain rights of Irish citizens in the United Kingdom, and designating the Republic of Ireland as its name for the resulting state. King George VI, who no longer had King of Ireland among his titles, sent a message of goodwill to the new Irish head of state, President Sean T. O'Kelly. O'Kelly's new status as head of state was celebrated by the first ever state visit by an Irish president abroad, to the Holy See in 1950. (En route, he planned to "do the decent thing and call upon Your Majesty" but timetabling problems prevented what was intended to be the first ever public meeting between a British king and an Irish president.)

The declaration of the republic had two controversial after-effects. On becoming a republic a country ceases to be a member of the Commonwealth of Nations. Though 1949 saw India as a republic reapply for membership and be accepted, the Republic of Ireland decided not to.2 More controversially, the British parliament's Ireland Act 1949 gave a legislative guarantee to Northern Ireland that Northern Ireland would continue to remain a part of the United Kingdom unless the parliament of Northern Ireland formally expressed a wish to join a United Ireland. This "constitutional guarantee" became a source of much controversy during the rest of the twentieth century.

The word Éire features on all Irish (and since 2002 Euro-Irish) coins and postage stamps issued since 1937. Before then, Saorstát Éireann, the Irish translation of Irish Free State, featured.

2 The issue of whether Ireland should rejoin the Commonwealth is occasionally raised. One of Sean Lemass's ministers, Brian Lenihan, suggested the Republic of Ireland should rejoin in the 1960s. The suggestion, previously approved by Lemass who wanted to see the reaction, drew a negative response and was quietly dropped. In the 1990s, Eamon Ó Cuiv then a junior minister (now a full cabinet minister), and coincidentally a grandson of Eamon de Valera, unilaterally suggested the Republic of Ireland should reapply for membership. The suggestion drew little hostility but no great enthusiasm. Ó Cuiv has continued to raise the issue occasionally.